диафрагмированные волноводные фильтры / efd6a9c2-453b-4f3f-b440-4347ab417939

Abstract — Unlike conventional filters and antennas that are separated, a filtering antenna with 3rd-order Chebyshev response is presented in this paper. At first, a 3rd-order Chebyshev bandpass filter is designed by utilizing coupling matrix. Then, an aperture is etched on the narrow wall of the last resonator of the filter for energy radiation. As demonstration, an example of the filtering antenna with center frequency of 9 GHz and bandwidth of 500 MHz is presented. Simulated results agree well with theoretical results. Compared with conventional filters and antennas, the volume of filtering antennas can be reduced to some extent.

Index Terms — filtering antenna, narrow-wall aperture antenna, Chebyshev filter.

I. INTRODUCTION

Conventional antenna topology is shown in Fig. 1(a), in which an antenna is connected to a filter to extract the certain band of the microwave.

In this paper, the author designed a novel structure called filtering antenna which works in a center frequency of 9 GHz, of which the last resonator not only works as a resonator of the filter but also an antenna [1]. It is designed based on the WR90 waveguide of which the dimension is 22.86 mm×10.16 mm. The coupling matrix theory is used to design the filtering antenna which is based on a 3rd-order Chebyshev filter [8].

Chebyshev filter coupling matrix theory is presented in details in this paper [2]. The author investigated the parameters of the coupling matrix such as Qe, Mij and Qr with realization of rectangular waveguide [7]. The author also researched how the parameters change with the variation of the dimensions. In this paper, the author designed the 3rd-order Chebyshev filter firstly [6]. Then with the approach of equal parameter replacement the author used an aperture antenna to replace the output port to make the whole filter a filtering antenna.

Fig. 1. (a) The traditional topology of the antenna which is connected to a bandpass filter. (b) The novel filtering antenna topology.

II. CHEBYSHEV BANDPASS FILTER DESIGNING

In this section, a Chebyshev bandpass filter is designed. For demonstration, its center frequency is 9 GHz and the bandwidth is 500 MHz. The return loss in the passband is set lower than -20 dB. According to coupling matrix theory in [2],

The external quality value and coupling coefficients could be computed:

Here, FBW=5.56% is the fractional bandwidth of the filter. Fig. 2 shows the result of theoretical S11 and S21.

Fig. 2. The theoretical chart of S11 and S21. The solid line is theoretical S11, and the dash one presents theoretical S21.

Fig. 3 presents the dimensions in the progress to design the filter [5].

Fig. 3. The structure of the 3rd-order Chebyshev filter. It is a symmetric structure. The waveguide is WR-90 which is 22.86 mm×10.16 mm. l0=10.16 mm and the width of iris t=1 mm.

A. Extraction of the External Quality factor

With the approach of extraction in [2], Fig. 4 presents the variation of Qe versus d1. It can be seen that when d1=15.06

Fig. 4. The variation chart of Qe versus d1. 11 is used to make sure the center frequency is 9 GHz.

B. Extraction of the Coupling Coefficient

According to [2], when d2 changes, we can get different value of M12 as Fig. 5 shows:

Fig. 5. The variation chart of M12 versus d2. 12 is used to make sure the center frequency is 9 GHz.

From the chart, we can find that M12 the increase with the value of d2 become larger. When d2 =12.50 mm, M12 could arrive the designed value 0.5731. At the time, the 12 should be 22.77 mm to keep the center frequency at 9 GHz.

After optimizing with CST, the response of the scattering parameter is shown in Fig. 6.

It can be seen that the S-parameters agree with the initial design very well. Table I presents the changes of the dimensions of the filter.

TABLE I

VARIATION OF THE PARAMETERS

Fig. 7. The designed structure of the filtering antenna. (a) is the internal structure of the filtering antenna of which is filled with air. Port 1 is the input port and Port 2 is the radiation port. (b) is the real structure with the roof opened. l0=10.16 mm, t=1 mm.

But firstly we should find the proper dimension of the antenna to get the desirable Qr. According to the method in [8], Fig. 8 presents the Qr value versus the variation of the aperture length lr.

Now we have found the proper initial dimensions of the filtering antenna. After simulation and optimization with CST, the initial and optimized responses are shown in Fig. 9:

Table II presents the variation of the dimensions after optimization.

TABLE II

VARIATION OF THE PARAMETERS

Fig. 10 shows the comparison between the S-parameter of the optimized structure and theoretical value.

Fig. 10. The optimized S11 response of the 3rd order Chebyshev filtering antenna compared with the theoretical response.

We can see that the simulation result is well agreed with the theoretical response computed with the coupling matrix.

Fig. 11 and 12 show the far-field realized gain of the filtering antenna:

Fig. 12. The beam pattern of realized gain in H-plane.

The realized gain of this design is 6.58 dB. It can be see that the beam pattern performs well in both E-plate and H-plate.

IV. CONCLUSION

In this paper, a filtering antenna with 3rd-order Chebyshev response is designed, whose center frequency is 9 GHz and the bandwidth is 500 MHz. Both simulated and theoretical results agree well. Compared with conventional filters and antennas, the volumes of filtering antennas can be reduced to some extent.

REFERENCES

[1]Chang, K., York, R. A., Hall, P. S., et al.: ‘Active integrated antennas’, IEEE Trans. Microw. Theory Techn., 2002, 50, (3), pp. 937–943

[2]Hong, J. S., Lancaster, M. J.: ‘Microstrip filters for RF/microwave applications’ (Wiley, 2001)

[3]Williams, Albert E. "A four-cavity elliptic waveguide filter." G- MTT 1970 International Microwave Symposium. 1970.

[4]Atia, Ali E., and Albert E. Williams. "Narrow-bandpass waveguide filters "Microwave Theory and Techniques, IEEE Transactions on 20.4 (1972): 258-265.

[5]Shang, Xiaobang. SU-8 micromachined terahertz waveguide circuits and coupling matrix design of multiple passband filters. Diss. University of Birmingham, 2011.

CST Microwave Studio. CST GmbH, Darmstadt, Germany, 2006.

[6]Nugoolcharoenlap, Ekasit. New design approach of antennas with integrated coupled resonator filters. Diss. University of Birmingham, 2015.

[7]Lancaster, Mike J. Passive microwave device applications of high-temperature superconductors. Cambridge University Press, 2006.

[8]Pozar, David M. Microwave engineering. John Wiley & Sons, 2009.